1. Field of the Invention
This invention relates broadly to apparatus and methods for detecting tuberculosis. More particularly, this invention relates to a single test apparatus which is capable of detecting tuberculosis in multiple animal species.
2. State of the Art
Tuberculosis (TB) is a serious disease caused when bacteria attack the respiratory system. Generally, TB is divided into three categories: human, avian, and bovine. Human TB is rarely transmitted to non-humans, although other primates such as monkeys are susceptible to human TB (Mycobacterium tuberculosis). Avian TB (Mycobacterium avium) is typically restricted to birds, although pigs and occasionally other animals have been found to be susceptible to avian TB. Bovine TB (Mycobacterium bovis), also called cattle TB, is the most infectious across species and is capable of infecting most mammals. Bovine TB is caused by the bacterium Mycobacterium bovis which is part of the Mycobacterium tuberculosis complex. Bovine TB is spread primarily through the exchange of respiratory secretions between infected and uninfected animals. This transmission is most common when animals are in close contact with each other; i.e., animal density plays a major factor in the transmission of M. Bovis. While bacteria released into the air through coughing and sneezing can spread the disease, research also suggests that bovine TB can also be contracted from ingesting contaminated food.
Tuberculosis is a chronic disease in which host animals may show no symptoms of infection. For cattle which are part of the human food supply, the United States has a nationwide surveillance program in place at both State and Federally inspected plants. These inspections are post-mortem inspections which look for indicative lesions on organs, intestines and lymph nodes. Additionally, States requiring testing for import or export may conduct a skin test which is an immune response to tuberculin which is derived from killed TB bacteria. Swelling and irritation around the site of injection of tuberculin would be indicative of a potentially infected animal. Further diagnosis would then be required to confirm.
In addition to TB testing of cattle, TB testing is desirable in zoos. One means for detecting TB is tuberculin skin testing (TST) where mammalian old tuberculin is intradermally injected into the skin of the animal. Following injection, the injection site is checked at 24, 48 and 72 hours for a hypersensitivity-induced induration, the appearance of which indicates previous TB exposure.
TST has several serious shortcomings. A first shortcoming is that the TST lacks sensitivity; i.e., there are many false negatives. False negative tests are known to occur in early or advanced disease states. In advanced states, the absence of delayed hypersensitivity may be attributed to anergy; the absence of sensitivity to substances that would normally elicit an antigenic response. Concomitant severe illness, viral infections, nutritional deficiencies, recent immunizations, may also result in false negative reactions.
A second shortcoming of TST is that TST also lacks specificity; i.e., there are false positives. A major cause of these false positive tests is believed to be cross-reactivity between some of the tuberculin antigens and nonpathogenic species of environmental mycobacteria. False positives are also known to arise from nonspecific inflammatory responses by an uninfected animal to a component of mammalian tuberculin.
A third shortcoming of TST is that the reading of the results in somewhat subjective. For example, the reading scale in the U.S. is different than the reading scale in the U.K. and the reading scale in South Africa. Further, because the mammalian old tuberculin itself varies from unit to unit, results are not uniform. The effect of this shortcoming is that in reading a result, it is standard to have three possibilities—positive, negative, and suspect.
A fourth shortcoming of the TST is that the test is labor intensive in that it requires at least two interactions with the animal; i.e., both an injection and an inspection. It also requires that the animal be available for both interactions, which is not always practical and cost-effective for herds and for wildlife.
Because of all of the shortcomings of TST, current testing protocols mandate multiple testing and quarantine of animal species imported into the United States. If any animal tests positive, all other animals from the same shipment must begin a new period of quarantine. In addition, in the U.S. and some other countries, suspect animals may be subjected to an interferon-gamma test which is specific to the species of animal and which is relatively expensive to carry out. However, the results of the interferon-gamma test are affected by the previous TST test, making the entire TB testing algorithm difficult and undesirably expensive and inaccurate.
Recognizing the shortcomings of TST and the effort and costs associated with preventing TB outbreaks, several entities such as the Center for Disease Control, the NIH, the U.S. Public Health Service, and others have called for the development of a quick and reliable test for nonhuman primate TB. In addition, as suggested above, since various State and Federal agencies require TB testing of cattle, it would be extremely useful and desirable to have quick and reliable TB tests for non-primate mammals. Further, since testing for many different animals is desirable in zoo and wildlife settings, it would be extremely useful and desirable to have a single test which could be utilized for multiple species and which is quick and reliable, rather than having separate tests for each different species.